The Playlist took part in a live Facebook chat with director Robert Rodriguez, who said that the long-awaited Sin City 2 will be his next movie after Spy Kids 4: All the Time in the World. Here's an excerpt from the article, which also revealed that this will only happen if the script is ready in time:

"Frank Miller and I are working on it," Rodriguez said, with one caveat. "It's slated to be the next one, but if the script is not ready, then we'll have to move to the next one."
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Robert Rodriguez wouldn't reveal which movie, "the next one" will be, although he did indicate that one of his other future projects, Nervewracker is, "still going to happen."

Although Bruce Willis' character, Hartigan, did die in Sin City, Rodriguez also indicated that he wants to bring the character back for Sin City 2:

"I wanna bring him back for Sin City because he's so bad-ass."